In rotorcraft, as exemplified by the tiltrotor rotorcraft of FIG. 1, the rotor hub 114 is at the heart of the rotor system 110 design, and central to the functioning of a rotorcraft. In the case of a tiltrotor 100, the rotor hub 114 transmits loads from a rotating rotor blade 116 to a tilting nacelle 112 carried by a wing 102. It is in the hub that additional rotor articulation degrees of freedom such as rotor blade feather, flap, or lag are accommodated.
There exist many different rotor configurations in the field of rotorcraft. The rotor configuration drives the hub configuration and requirements. In the case of an aircraft with a propeller, bending out of the rotating plane of the propeller is usually low, and axial thrust is high. Typically the propeller rotational speed, expressed in revolutions per minute (RPM), is significantly higher than that of a helicopter rotor, requiring a small diameter propeller shaft in order to keep the bearing tangential speeds within material and lubrication limits.
Unlike typical propellers, a helicopter rotor can change the feather (pitch) angle of blades in a non-uniform manner. Thus, for teetering, gimbaled, or articulated rotor types, a helicopter rotor can change the direction of (vector) the thrust of the rotor blade and rotor without vectoring the direction of the hub itself. FIG. 2A illustrates a typical articulated rotor helicopter 200, while FIG. 2B illustrates the rotor hub system 210 of the helicopter. Rotor blades 212 are connected to a rotor mast 214 and hub structure 216 by a flap hinge 218, pitch bearing 220, and lag hinge 222. These hinges allow the blades to move independently of the hub.
For hingeless or bearingless rotor types there are no flap or lag hinges, and a helicopter rotor can transmit some bending moments to the rotor mast, which supports the hub and rotor. FIG. 3 illustrates a hingeless rotor hub system 300, in which a rotor blade 302 is connected to a pitch bearing and housing 304 that attaches to a hub 306 rotating mast 308. Helicopters usually operate at a rotor RPM much lower than that of a propeller; in either case the product of diameter and RPM is usually such that the tip approaches sonic speeds at some flight conditions. The ability to transfer bending moments necessitates a stiffer rotor to mast connection, which must be accommodated by the rotor hub. Most often, the rotor hub transfers the axial and bending loads down a small diameter mast structure into an internal frame structure or gearbox, and that structure is in turn mounted to the airframe.
FIG. 4A illustrates a helicopter 400 with a teetering rotor system 410, while FIG. 4B shows additional detail on the teetering rotor and hub system 410. For this type of hub system 410, the teetering rotor is attached directly to a long mast 412 having a small diameter. The mast 412 couples the non-rotating helicopter structure 402 to the helicopter blades 414. Blade pitch change is accommodated with a feather bearing 416. The rotor hub system 410 is hinged with respect to the mast 412 by means of a teetering hinge 418. The teetering rotor allows the rotor to vector the thrust direction, but produces only very small moments in the rotor mast. Any mast loads are commonly resisted by an internal structure with bearings whose diameter is a small fraction of that of the dimensions of the airframe structure to which it attaches.
In the case of a hingeless rotor, the hub structure takes bending moments as well as provides for rotor blade feathering. Hingeless rotor hubs exist in many applications. FIG. 5 illustrates the prior art of the Sikorsky™ X2 demonstrator hingeless coaxial rotor hub system 500, which includes upper 510 and lower 520 rotor hubs that couple blades 530 to non-rotating structure 540. Blade feather bearings in housings 512 allow the blade to pitch; the blade feather bearings are placed at a distance approximately two times the hub bearing 514 diameter.
FIG. 6 illustrates the prior art of a Eurocopter™ Bo 105 rotor hub 600. The Bo 105 rotor hub 600 is machined from titanium and houses bearings 610 that allow blade 620 feathering. This hingeless rotor hub 600, like other prior art hingeless rotor hubs, does not and cannot absorb very large bending moments on the order of hundreds of thousands of foot-pounds of moment. Additionally, most hingeless rotor hubs are used in helicopter rotor applications where the total angular travel of blade feather is less than 40 degrees total. In this manner, many of these applications are able to use flexible elements for the feathering joint. Currently the rotorcraft industry trends toward hingeless rotor designs with lower total part count and fewer moving parts. Elastomeric bearing elements and flexible beam elements are common.
In applications where the rotor blade feathering motion is supported by a rolling element joint, lubrication for this joint is usually permanent, such as grease, and requires no continuous cycling of lubricant. Consequently, the real-time means by which to monitor the joint health is by temperature sensing. The flow of lubricant through a rolling element joint would otherwise allow for bearing health monitoring by means of a chip detector and/or temperature sensor. In some cases, a rolling element feathering joint such as (U.S. Pat. No. 5,387,083 to Larson) is fed by a lubrication distribution and cooling system included in the rotating frame of the hub. Due to this lack of hub bearing lubrication systems, or the isolation of the cooling system in the rotating frame, current hub systems can be detached in the field from the gearbox assembly that drives them. Thus, the gearbox and rotor hub system of conventional rotorcraft are separate field replaceable units.
The '083 patent, and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Outside the field of rotorcraft, designers have developed different rotor hub designs to deal with issues specific to those other fields. In the field of large windmills, for example, efficient use of structure is a major focus. U.S. Pat. No. 7,335,128 to Flamang et al. proposes the use of a rotor hub-to-nacelle coupling that directly transfers the bending and in-plane loads from the rotor into the nacelle structure. The gearbox in that design is isolated from the bending and in-plane load path, coupled only torsionally to the rotor hub.
Flamang '128 proposes a coupling arrangement from a windmill rotor hub to nacelle windmill nacelle. As will be appreciated by those skilled in the art, a windmill generally seeks, by rotating the nacelle into the wind, to run in stable axial flow conditions through the rotor, and therefore has comparatively low blade and hub bending moments. The bearing arrangements from Flamang '128 indicate that the loads on the windmill rotor are predominantly in the plane of the rotor.
Unlike most hingeless rotors, an Optimum Speed Rotor (OSR), U.S. Pat. No. 6,007,298, or an Optimum Speed Tilt Rotor (OSTR), U.S. Pat. No. 6,641,365, are capable of higher moments at the hub. The initial implementation of OSR technology, in the Boeing™ A160, used a relatively small 36 foot diameter rotor. An especially preferred embodiment referenced in the OSTR patent is a 30 foot diameter rotor.
An OSTR rotor, like all rotors, is subject to dynamic aeroelastic loads, and is generally designed to avoid instabilities. As a rotor design is scaled up, such dynamic solutions dictate an increase in rotor weight proportional to rotor diameter cubed. This would remain true unless a different and lighter dynamic solution is applied for a larger rotor. Rotor weight has a substantial effect on rotorcraft empty weight and therefore on the useful load (payload and fuel) per rotorcraft size and cost.
At a constant disc loading, the lifting capability of the rotorcraft is proportional to the rotor diameter squared while the rotor weight is proportional to the rotor diameter cubed. This is known as the square-cube law in the industry and results in both an undesired trend of increased disc loading in larger rotorcraft, and extreme difficulty in designing very large rotorcraft. Because of the high bending loads associated with hingeless rotors, hingeless rotors have been historically flexible, and limited to small rotors in order to avoid the increase in rotor weight resulting from these loads. Large rotor designs often increase disc loading in order to reduce the diameter required for a given vehicle weight. Furthermore, prior art large rotor designs have articulated hub systems to minimize blade flap and hub bending moments.
If rotorcraft with stiff, hingeless rotors were to be conceived at larger scales and higher disc loadings, the approach to obtaining lightweight and stiff rotor designs would necessarily diverge from the embodiments described in the '298 and '365 patents. One aspect that would need to be engineered is the reduction of deflections of the bearing supporting areas of the hub structure. For long life rolling element operation, the bearing support structure must remain relatively planar and round. Although these goals are not unique to rotor hub designs, see for example U.S. Pat. No. 7,244,102 to Delucis, the magnitude of loads and relative nature of the concept of what constitutes lightweight in proportion to size are completely different between rotorcraft and land-based wind turbines.
Therefore, what is needed in the rotorcraft industry is a high moment capable hub structure for a rotorcraft, having sufficient strength and stiffness to operate under very large bending moment loads, on the order of hundreds of thousands or millions of foot-pounds of moments.